1. Field of the Invention
The present invention relates to a method for protected transmission of digital, or digitized, respectively, signals via radio circuits, wherein the transmission of the signals, possible encoded, proceeds in a blockwise manner (burst operation) with blockwise pseudo-randomly alternating radio carrier frequencies (frequency hopping), and wherein the successive signal blocks are respectively provided with an error protection, and wherein the synchronization between two stations is brought about in conjunction with highly-accurate quartz clocks arranged at the stations, by means of a time signal in the form of a code sequence having good auto-correlation properties, which time signal is transmitted at least at the beginning of a signal transmission.
2. Description of the Prior Art
Methods of the type generally set forth above are known in the art, for example, from the German patent 32 30 726 A1. As has been shown, for the operational protection of a radio transmission path, the diversity techniques which have been employed for decades in conjunction with error protection measures no longer suffice when the transmission path is electronically jammed in an intentional manner. In order to be able to combat such intentional interference successfully, it is necessary to alter the radio frequency carrier employed for the signal transmission in a pseudo-random manner in a sequence which is as rapid as possible. In the case of such a frequency hopping operation, in order to obtain the signal pauses necessary for switching the radio carrier frequencies, the signals to be transmitted which are present on the transmitting side are first compressed in time and then transmitted to the receiving side in successive signal blocks with a radio carrier frequency alternating from signal block-to-signal block. Customarily, each signal block is provided with an error protection, so that, at the receiver, every signal block can be checked as to whether it is utilizable for further interpretation or if it must be rejected as disturbed. The synchronization between two end stations, necessary for satisfactory transmission, is achieved through highly-accurate quartz clocks at the stations in a simple manner in that, at least at the beginning of a transmission from the transmitting station to the receiving station, a time signal is transmitted. With the aid of such a time signal, the cryptographic equipment, provided at the station in the case of signal encoding, is also indirectly synchronized.
The satisfactory operating properties of a radio transmission path are generally impaired by so-called fadings which are brought about by superimposition of several radio signals originating from the same source. In the use of mobile radio stations, strong fluctuations of the receiving field intensity can also be caused by locally-induced shadowings. The negative influence of such fadings, as well as of the shadow areas, on the signal reception are mitigated in that radio waves, as a rule, propagate on several paths, and therefore, in most instances, a signal reception, even if with substantially less receiving field intensity, is also still possible when the receiving path on which the reception is being carried out at the time intermittently fails entirely. In the case of application of a frequency hopping operation, this phenomenon of multipath propagation has an extremely disadvantageous effect on a satisfactory operation of a radio path because the sum levels also vary in dependence upon the employed radio carrier frequencies, and therefore, frequency-selective fadings of this type lead to the result that the receiver is forced to receive the successive incoming signal blocks because of the constantly alternating radio carrier frequencies, in dependence upon the radio carrier frequency, on different transmission paths. Since the different transmission paths can exhibit different transit times which are greater than half a bit, the incoming signal blocks, in spite of bit clock pulse synchronization, can occasionally no longer be correctly processed. In other words, the signal block phase can jump from signal block-to-signal block by one or more bit intervals depending upon whether the incoming signal block reaches the receiver on the direct transmission path or over a substantially longer, indirect route.